The present invention relates generally to resistance of polycrystalline materials to intergranular degradation or failure (IGDF), and particularly to disruption of the material""s random grain boundary network connectivity (RGBNC) structure as an indicator of the material""s resistance to IGDF. This indicator may be used to assess the effectiveness of engineering processes to increase the material""s IGDF resistance or as a diagnostic tool to detect possible onset of material failure due to IGDF.
The phenomenon of stress corrosion cracking (SCC) in structural materials due to the collective actions of stress, material microstructure, and environment have been recognized for many years, and the mechanisms have been extensively investigated. The SCC process is believed to be governed by the subprocesses of crack initiation and crack propagation. The method of grain boundary engineering is currently seen as one means of modifying materials in order to increasing the SCC resistance of the grain boundaries.
A grain boundary is formed where two single-crystal grains in a polycrystalline aggregate meet. The boundary is characterized by its macroscopic and microscopic degrees of freedom. In its ideal form, the boundary is planar and defined by the misorientation of the grains on either side of the boundary (two degrees of freedom for the axis of misorientation and one for the misorientation angle) and the plane of the interface (two degrees of freedom). The rigid-body shifts, parallel and perpendicular to the boundary plane, comprise the three microscopic degrees of freedom. The general grain boundary is not planar and can take on curvatures consistent with the energetics of the system.
It is common practice to describe grain boundaries by the misorientation of one grain with respect to another. It is convenient to use the axis-angle notation to denote rotation axis and the rotation angle necessary to transform one into the other. Consider two crystal lattices misoriented with respect to each other and allowed to interpenetrate. At certain axis-angle pairs, the lattices form special patterns characterized by the coincident site lattice (CSL) notation (described by H. Grinrner, Acta Crystallographica Section Axe2x80x94Foundations of Crystallography, A30 (1974)680). In this notation, the misorientation is denoted as xcexa3n where n is the reciprocal density of coincident lattice sites, n is always odd. The CSL notation is geometrical only and Ad disregards the plane of the grain boundary and the microscopic degrees of freedom. Although one would not expect macroscopic properties to correlate with xcexa3n, however there is strong evidence that such a correlation exists for some properties.
Grain boundaries are often grouped into broad classes, such as low- and high-angle, twist and tilt, and special and random. The first class is based on structure and energy criteria while the second and third classes are strictly geometrical in nature. Conventionally, the delimiting angle separating low- from high-angle boundaries is 15 degrees for cubic crystals. This is approximately the angle where it is no longer possible to discern well-separated dislocations forming the boundary.
Strictly speaking, special boundaries (boundaries that have low xcexa3 and exhibit special properties) occur at well-defined misorientations, but it has been shown that boundaries near an exact xcexa3 misorientation can exhibit xcexa3-like properties. The acceptance angle, xcex94xcfx86, over which boundaries exhibit xcexa3-like properties is usually expressed as:
xcex94xcfx86=xcex94xcfx860xcexa3xe2x88x92m
where the prefactor xcex94xcfx860 is 15 degrees, and m=1/2 according to the Brandon criterion [D. G. Brandon, Acta Metallurgica, 14 (1966) 1479].
Not all boundaries that meet this criterion exhibit special properties. Generally speaking, special boundaries are those boundaries with xcexa3xe2x89xa629. Other boundaries, including xcexa3 greater than 29 are considered random. This arbitrary cut-off value of xcexa329 for cubic crystalline materials, was first suggested by Watanabe [Watanabe, T., J. Physique, 1985, 46(C4), 555]. The distribution of boundary types with respect to xcexa3 is called the grain boundary character distribution (GBCD).
Many important physical and mechanical properties of materials are intimately coupled to microstructural features such as chemistry, grain size and shape, texture, and the presence of second phases and precipitates. It is possible to tailor the microstructure of metals alloys through thermomechanical processing to obtain orders of magnitude improvement in resistance to corrosion, stress corrosion cracking, creep and possibly to irradiation assisted stress corrosion cracking. These processing methods have generically become known as grain boundary engineering.
In grain boundary engineering, properties such as those described above have been found empirically to correlate with the fraction of xe2x80x9cspecialxe2x80x9d boundaries in the microstructure. Palumbo (G. Palunbo, U.S. Pat. Nos. 5,817,193 and 5,702,543) has in described methods by which a material can be processed to increase the fraction of special grain boundaries in a microstructure. This typically involved sequential thermomechanical processing (TMP) where a material is deformed by a moderate amount, e.g. 20% and annealed at a relatively high temperature for a relative short time. The process of deformation and annealing is repeated until the desired special fraction is obtained.
In a few documented cases, intergranular stress corrosion cracking (IGSCC) has been observed to propagate along the interconnected random grain boundary network. Adams et al [Y. Pan, B. L. Adams, T. Olson, and N. Panayotou, Acta Materialia 44 (1996)4685] have analyzed crack path dependence of IGSCC of alloy X-750. The study examined some 818 cracked triple junctions. The choice of which boundary the crack advances upon was studied as a function of misorientation and inclination relative to the stress axis. The general observation is that random boundaries are most susceptible to cracking when the direction of forward propagation of the crack lies within an angular range of xcx9c20 degrees about the crack plane. Low angle (xcexa31) and xcexa33 boundaries are observed not to crack for any plane inclination. Some CSL boundaries lying in the range xcexa35-xcexa349 did crack; however, when the plane inclination was considered, boundaries whose planes lie sufficient close to the coherence plane(s) were observed not to crack. Watanabe [Watanabe, Res Mechanica, 1984, 11, pp 47-84] states that low-angle and coincidence high-angle boundaries are resistant to segregation-assisted IG fracture, whereas random high-angle boundaries are preferential sites for IG fracture in most situations.
It has been found that properties that are favorably influenced by grain boundary engineering tend to have percolative mechanisms, which depend on the topology of the grain boundary network. Wells et al. [Wells, D. B., Stewart, J., Herbert, A. W., Scott, P. M. and Williams, D. E., Corrosion, 1989, 45, 649], on the basis of a bond percolation formulation, suggested an appropriate statistical function that would describe when the assembly of grain boundaries in the microstructure attained a critical value of active segments. On the basis of these simulations, Wells predicted that the minimum fraction of random boundaries in a three-dimensional lattice structure that would lead to the formation of a one-dimensional continuous linear chain was 0.23. However, when a planar section, based on an approximation of the two-dimensional microstructure to a honeycomb network, was considered then this boundary fraction reached a value of approximately 0.65. This suggests that the probability of cracks propagating through the microstructure would be considerably reduced as the special fraction increases beyond 0.35.
Advances in the engineering of grain boundaries in materials have been facilitated in recent years by a scanning electron microscope (SEM) technique, known as electron backscattered diffraction (EBSD), for automated indexing of electron backscattered diffraction Kikuchi patterns. This technique has largely superceded other experimental techniques, such as transmission electron microscopy (TEM) and electron channeling in the SEM, for the determination of the GBCD due to the relatively straightforward specimen preparation and the large number of orientation measurements attainable in a relatively short period of time. Thus, advances in the engineering of grain boundaries can be ascribed due to the following factors: (1) recognition that grain boundaries play an important role in a number of materials properties, (2) recent evidence that TMP can alter the GBCD, and (3) ease of characterization of the GBCD by the automated EBSD patterns technique.
The SEM-based set-up automatically acquires and processes EBSD patterns for determination of local orientations, misorientations, and microtexture. It allows the orientation at spatially specific points in planar sections of the microstructure to be measured and directly correlated with results from other imaging techniques such as optical or scanning electron microscopy. The acquisition of an EBSD pattern requires a highly collimated, stationary electron probe focused on a steeply inclined specimen. The interaction of the electron beam and the specimen generates an EBSD pattern by the backscattering of electrons from favorably oriented crystal planes. Individual orientation measurements are made at discrete points on a sample; the locations of the points are defined by a grid of dimensions prescribed by the user (both in the width and height of the grid as well as the spacing between points on the grid). At each point in the grid, the backscattered Kikuchi diffraction pattern is captured, frame averaged and automatically indexed. The three Euler angles that describe the orientation are recorded along with coordinates describing the position. Thus, images (or maps) can be generated by mapping the crystal orientation onto a color or grayscale and shading each point on the grid according to some aspect of the crystal orientation. Alternatively, misorientations between points can be indicated by drawing boundaries that are color coded by type of boundary, as for example, special or random.
An object of the present invention is a method for determining the resistance of polycrystalline materials to intergranular degradation or failure (IGDF), by analysis of the random grain boundary network connectivity (RGBNC).
Another object of the present invention is a method for confirming the improvement in IGDF resistance of polycrystalline materials that have been subjected to materials processing.
Another object of the present invention is a method for inspecting existing polycrystalline material structures operating under extreme conditions (such as high temperature, high stress or corrosive environments), to detect susceptibility to potential intergranular degradation and/or failure. The method comprising comparing stereological parameters from the RGBNC for the existing stressed material to the analogous parameters from the pre-existing unstressed material""s RGBNC.
As discussed earlier, conventional grain boundary engineering efforts currently focus on increasing the ratio of xe2x80x9cspecialxe2x80x9d to xe2x80x9crandomxe2x80x9d grain boundaries. However, the inventors conclude that merely increasing the ratio, although necessary is not sufficient to improve properties such as resistance to intergranular degradation or failure. In fact, IG degradation and failure, which include IG corrosion and IG fracture, are primarily dependent on the spatial distribution and interconnectivity of the boundaries prone to crack propagation, i.e. the grain boundary network. In particular, the degree to which the random grain boundary network connectivity (RGBNC) has been disrupted by the special boundaries may be used as a key indicator of a material""s IGDF resistance. Assessment of the RGBNC after a material has undergone processing, such as thermomechanical working, can indicate whether the material""s IGDF resistance has improved. Alternatively, comparison of the RGBNC for a material in service under extreme operating conditions (high temperature, high stress, highly corrosive environment, or combinations of the foregoing) against the RGBNC for the same material unexposed to the extreme conditions can provide an indicator whether the service material might undergo failure due to an IGDF mechanism.